PayPal Website Payments Pro Integration Tutotial

I have a website where I want people to be able to pay their bill online. Along with the normal information to make payments, they need to be able to add their customer ID. Also, they need to be able to enter the amount they want to pay as everyone's bill will be different.

I do not need you to do all the work. Just walk me through a step by step process. I am quite knowledgeable but am having the hardest time understanding the exact process.